1. Field of the Invention
The present invention generally relates to hand-held measuring devices and, more particularly to a new and improved measuring device adaptable for easily determining both major and pitch diameters of screw threads, gears, splines and the like.
2. Description of the Prior Art
The use of three gauging wires of known diameter to determine the pitch diameter of a screw thread, gear or spline is well known in the art. As discussed in U.S. Pat. No. 3,068,582 issued to Crossley, the method involves positioning two gauging wires side-by-side in adjacent grooves of the screw thread to be measured. The remaining gauging wire is positioned in a groove diametrically opposed from the first two wires and an "over the wire" measurement is taken by an outside micrometer. The actual pitch diameter is then easily determined by subtracting from the micrometer measurement a constant associated with the particular gauging wires employed.
It is extremely difficult to hold the three gauge wires in their proper positions against the screw thread while, at the same time, positioning the micrometer to make the over-wire measurement. Several such measurements often must be made over the length of the screw thread to determine size, taper and roundness of the pitch cylinder. In practice, gauging wires are often dropped and lost during the measuring process.
A variety of devices have been suggested for maintaining gauging wires in their proper relative positions during the screw thread measuring process. For example, the above cited Crossley U.S. Pat. No. 3,068,582 employs a separate, notched holder which, itself, can be easily misplaced when not in use. Furthermore, it would be necessary to maintain notched holders of varying sizes to accommodate screw threads of varying diameter. A further device is suggested in U.S. Pat. No. 2,663,948 issued to Scully, wherein a plurality of separate gauging wires are each mounted on a cylindrically-shaped holder. The Scully device employs a rather complicated and awkward adjustment collar and screw assembly for adjusting the relative positions of the gauging wires.
In an apparent effort to overcome the problems associated with the need for separate holders, Japanese Pat. No. 57-149902 issued to Nakarnishi suggests a process wherein a screw thread to be measured is temporarily magnetized to retain a plurality of gauging wires in place by magnetic attraction.
Because this procedure can only be effective when employed with screw threads formed of magnetizable material, Nakarnishi does not solve the problem of retaining gauging wires in their relative positions when measuring the pitch diameter of screw threads formed of such common materials as aluminum, brass, stainless steel, plastic or the like. Nakarnishi also fails to suggest any structure for calibrating and adjusting the gauging force as required to compensate for gauging wires of various size and shape.
While three gauging wires are generally considered the most acurate means of determining pitch diameter, other gauging devices such as that suggested in U.S. Pat. No. 2,440,967 issued to Moore are well known. Moore does not teach the use of three gauging wires. Rather, Moore suggests a gun-shaped device having a first gauging block holding two gauging wires and a further, curved gauging block. The blocks are biased into contact with opposite sides of the screw thread and an integrally mounted indicator is positioned for directly displaying the pitch diameter. Devices such as suggested by Moore are difficult to calibrate and can only be used where the curved surfaces of the gauging blocks precisely match the circumference of the screw thread. As it becomes necessary to measure screw threads of varying diameter, it is evident that the Moore device must repeatedly replace both gauging blocks to maintain proper engagement with the screw thread. In a similar manner, the pair of gauging wires must also be changed to assure proper fit. It is further noted that the gauging pressure generated between the blocks and the screw thread in Moore is dependent upon the continuous pressure force applied to the trigger. Because it is difficult, if not impossible, for an operator to apply constant pressure to the trigger, the gauging pressure tends to fluctuate, often leading to an improper reading by the measuring device.
As will become clear hereafter, the present invention provides a measuring device which overcomes the referred to problems confronting known prior art assemblies, while at the same time providing an effective and accurate procedure for measuring pitch diameters of screw threads, gears, splines and the like.